In-situ selective oxidation created Cr₂O₃ assisting CrMnFeCoNi for ultrahigh power density zinc − air batteries

Highly efficient and stable bifunctional catalysts toward sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are critical for practical applications of rechargeable zinc-air batteries (ZAB). Heterostructure engineering is effective in boosting catalytic performances of the bifunctional catalysts. Here, N-doped carbon supported quinary alloy-chromium oxide heterostructured catalysts, CrMnFeCoNi-(CrO_x)₁/NC, were created, through in-situ selective oxidation of Cr, to construct binder-free composite air electrodes for high performance rechargeable ZABs. The CrMnFeCoNi-(CrOx)1/NC based ZAB exhibited outstanding discharge-charge performances, delivering an ultrahigh discharge peak power density of 364 mW cm^-2 at 499 mA cm^-2, an ultra-narrow working voltage gaps of 0.78 V at 10 mA cm^-2 and 0.87 V at 50 mA cm^-2, and ultrastability of 765 hours at 10 mA cm^-2 and 100 hours at 50 mA cm^-2, which largely outperformed the (Pt/C+RuO₂) based one. In-situ Raman and XAS studies revealed that Mn, Fe, and Co were the main active sites for ORR, whereas Mn, Co, and Ni played the key roles in catalysis of OER. The presence of Cr₂O₃ offers abundant oxygen vacancies, beneficial for enhanced oxygen adsorption to boost ORR. Further oxidation of the oxidation-prone Cr₂O₃ to CrO₃ during OER is advantageous for protection of OER-active intermediates from over-oxidation to enhance OER.